The third generation partnership project, 3GPP, is currently working on specifying support for HSDPA flow control, HSDPA FC. The goal of HSDPA FC, is to avoid and/or resolve congestion in a transport network, TN, between RNC and NodeB (i.e. a radio base station, RBS) and to keep enough data packets in the packet queues, PQ, in NodeB while at the same time avoiding unnecessarily long delays in the flow of data in the transport network.
In the prior art there are several ways in which this is done. For example, an early attempt was rate based aggregated flow control. However, this technique has been surpassed by later developments.
Another example is rate based per-flow flow control. When so-called active queue management, AQM, based congestion control, ABCC, is introduced the use of rate based per-flow flow control will remain. One of the reasons is that ABCC cannot support asynchronous transport mode, ATM, based transport networks, whereas rate based per-flow flow control can be used also in ATM based transport networks. Per flow (i.e. per radio bearer) bitrate is calculated based on Iub (the interface between RNC and NodeB) congestion detection and based on the status of the high speed medium access control, MAC-hs, scheduling buffer in the NodeB. The calculated bitrate is signaled from the NodeB to RNC. In the RNC MAC-d shapes the flow according to the received bitrate.
ABCC will be available in parallel with rate based per-flow flow control. This solution is intended to be the mainstream solution when the radio access network, RAN, and the transport network (Iub) is Internet protocol, IP, based. In the future it is expected that communication network operators move towards IP-based transport. Therefore it can be important to support all features with ABCC as it is expected to be a mainstream solution in the future.
ABCC intends to re-use end-to-end transmission control protocol, TCP, for congestion control both over Iub and for buffer control of the MAC-hs (priority queue, PQ, buffer) in the NodeB. This behavior is achieved by destroying end-user IP packets, when Iub congestion happens or when the MAC-hs buffer grows too long. In case of this solution Capacity Allocation Control Frames are either not sent, or even if sent they indicate very large bitrates, most likely larger than the actual bitrate achieved by the radio bearer, RB.
The main idea of multi-point HSDPA is that a user equipment, UE, receives packets from more than one radio cell, i.e. along different links or legs. In this way e.g. the cell edge user's throughput can be improved. For details see, e.g., figure 9.2 in 3GPP TR 25.872.
As mentioned, in ABCC the application level TCP is re-used as TN congestion control and PQ buffer control. Using ABCC for multi-link HSDPA, when congestion occurs on one of the links the TCP congestion avoidance mechanism is triggered and TCP will react on flow bitrate decrease. However, a consequence of this TCP congestion mechanism being triggered is that the UE cannot fully utilize the aggregated capacity of the links.